If so much mail is sent between the sponsoring organisations that the aggregate hit rate will be approximately 100%, then wouldn't approximately 100% of mail be sent over the TLS only port were the sponsoring organisations to decide to only use TLS among themselves (e.g. a hardcoded SMTPS everywhere list) ?
The point of an open standard is so that the vanishingly small fraction of people running their own mail servers can easily implement it too.
The point of an open standard is so that the vanishingly small fraction of people running their own mail servers can easily implement it too.